The current invention is directed to the delivery of a bias voltage using a pair of differential signals while it minimizing the electromagnetic interference produced by these differential mode signals in an Ethernet data port or other serial data port where a common mode transformer is employed for high potential (hipot) isolation.
Isolation transformers and common mode transformers are commonly used in the serial interface ports in communications equipment. One such serial interface type is Ethernet, as described in IEEE standard 802.3. In this type of communications interface, it is desired to transmit differential mode voltages over long distances to a remote point, and a pair of differential transmit signals and a pair of differential receive signals are sent over twisted pair copper wire. The data rate carried over these differential pairs varies from a data rate of 10 Mbps (million bits per second) to a data rate in excess of 1 Gbps. It is often desired to reduce the high frequency common mode component of the transmitted signal, and a class of transformers known as common mode transformers is employed, whereby the opposing flux produced by common mode currents cancels the common mode voltage at the output of the transformer, while the additive flux produced by differential mode currents reinforces the output voltage. In addition, an isolation voltage requirement provides for the biasing 1500V of DC from any differential pair to ground, and an isolation transformer is commonly employed to provide this isolation. FIG. 1 shows one such prior art termination. A device known as a SERDES (serializer-deserializer) provides a serial stream of differential output data on lines 41 and 41xe2x80x2, which couples to transmitter isolation transformer 14 of transformer 12. Isolation transformer 12 serves to provide the required hipot (high potential) isolation of typically 1500V between the data equipment associated with the serdes 40, and the incoming data lines of connector 30. Transmit isolation transformer 14 is followed by common mode transmit transformer 20 of common mode transformer 18. A common mode transformer provides very little impedance to currents of opposite polarity, and a higher impedance to currents of the same polarity, thereby reducing the conduction of high frequency currents from the serdes 40 to the external conductors of the cables attached to connector 30. This reduction in common mode transfer from the serdes to the connector conductors reduces the emissions of EMI (Electro-Magnetic Interference) from the equipment. A balancing transformer 26 provides for symmetry in the output voltages, as well as an impedance reference for the common mode transformer 20. The output of transmit common mode transformer 20 is also furnished to an output connector 30, which for Ethernet is pins 1 and 2 of a shielded RJ-45 connector. Similarly, the receive path comprises a differential input arriving on connector 30 pins 3 and 6, thereafter being furnished to common mode receive transformer 22 of common mode transformer 18, and to receive isolation transformer 16 of isolation transformer 12, and to the receive inputs 43 and 43xe2x80x2 of the deserializer section of serdes 40. Receive balancing transformer 28 of balancing transformer 24 provides an impedance reference for common mode choke 22, as before. The reference resistors 42 and 44 provide an impedance reference for the unbalanced voltages present in the center tap of the balancing transformers 26 and 28. Capacitor 50 is a high voltage type that provides an AC return path for unbalanced common mode currents generated by either receive balancing tranformer 28 or transmit balancing tranformer 26. Resistors 46 and 48 provide a high frequency termination for unused pins 4, 5, 7, and 8 of the RJ-45 connector 30, for the case where an 8 wire cable using 2 pairs of twisted pair conductors is employed. As the cable connected to connector 30 may be very long, common mode voltages appearing on the twisted pairs for transmit and receive capacitively couple to the other twisted pairs, so all wires of the cable are provided a high frequency reference to ground, including the unused pins 4, 5, 7, and 8.
A first object of the invention is to provide a bias voltage using two differential pair electrical connections while preserving the hipot characteristics of the prior art termination. A second object of the invention is to provide an operating voltage over two differential pair electrical connections while preserving the common mode EMI attenuation of the prior art termination. A third object of the invention is to couple residual transient noise energy on the twisted pair cables to a shield conductor.
A differential mode termination includes a transmit isolation transformer coupled to a transmit common mode transformer, which is coupled to an output connector. The transmit common mode transformer has a reference autotransformer with a center tap for providing a shield reference, as well as the application of a bias voltage. The differential mode termination also includes a receive isolation transformer coupled to a receive common mode transformer, which is coupled to an output connector. The receive common mode transformer is coupled to a reference autotransformer with a center tap for providing a shield reference to the common mode transformer. By providing a bias voltage to the transmit reference autotransformer center tap and a bias voltage return to the center tap of the receive reference autotransformer, it is possible to power remote equipment while minimizing common mode coupling of EMI from the serdes and other noise sources internal to the equipment to the conductors of cables attached to the output connector.